doc: Adds a doc on setting up Octo. (jitsi#719)

doc/octo.md

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+# Setting up Octo (cascaded bridges)
+
+## Jitsi Videobridge
+Add these properties to `/etc/jitsi/videobridge/sip-communicator.properties` to
+enable support for Octo:
+```
+org.jitsi.videobridge.octo.BIND_ADDRESS=10.0.0.1 # the address to bind to locally
+org.jitsi.videobridge.octo.PUBLIC_ADDRESS=1.2.3.4 # the address to advertise (in case BIND_ADDRESS is not accessible)
+org.jitsi.videobridge.octo.BIND_PORT=4096 # the port to bind to
+org.jitsi.videobridge.REGION=region1 # the region that the jitsi-videbridge instance is in
+```
+
+You need to make sure that all of the bridges can communicate via the socket
+addresses described in the properties above, and that the network is secure.
+
+## Jicofo configuration
+To enable the use of Octo in jicofo, you need to set the "selection strategy" by
+setting this property in `/etc/jitsi/jicofo/sip-communicator.properties`:
+```
+org.jitsi.jicofo.BridgeSelector.BRIDGE_SELECTION_STRATEGY=RegionBasedBridgeSelectionStrategy
+```
+
+The `RegionBasedBridgeSelectionStrategy` matches the region of the clients to
+the region of the Jitsi Videobridge instances. That is, it always attempts to select a bridge
+in the region of the client.
+
+The `SplitBridgeSelectionStrategy` can be used for testing. It tries to select a new bridge 
+for each client, regardless of the regions. This is useful while testing, because you can 
+verify that Octo works before setting up the region configuration for the clients.
+
+
+## Jitsi Meet configuration
+The last step in enabling bridge cascading is enabling the feature in the
+clients (in `config.js` (/etc/jitsi/meet/)):
+```$xslt
+testing: {
+    octo: {
+        probability: 1
+    }
+}
+```
+
+Values other than 1 can be used for an A/B test (e.g. use 0.5 for a 50% probability).
+
+
+## Configuring client regions
+Clients learn about the region that they are in via the value of
+`config.deploymentInfo.userRegion`. Thus, in order for Octo to actually work
+for geolocation, `config.js` must be served with the correct values. The best
+way to implement this is outside the scope of this document, and it depends on
+the environment in which Jitsi Meet is installed.
+
+One easy solution can be to set the variable based on the domain name, and then
+access the conference through the desired domain name, e.g. use
+`us-east.jitsi.example.com`, `us-west.jitsi.example.com`,
+`eu.jitsi.example.com` and have them serve custom versions of `config.js`.